1. Field of the Invention
The invention relates to a method and arrangement for actuating a wavelength-tunable laser diode in a spectrometer, where a power-time function is predetermined, in accordance with which the laser diode is tuned periodically over a wavelength range by virtue of a current profile with which the laser diode is driven being determined from the power-time function and measured values obtained from the laser diode.
2. Description of the Related Art
A method and arrangement are known from DE 41 10 095 A1. In this document, the measured values are provided by a monitor diode that detects the radiation power of the laser diode. As a result, the optical power-current characteristic is linearized and thus the offset is reduced.
In laser absorption spectroscopy, the light of a wavelength-tunable laser diode is passed through a sample gas, and the concentration of a gas component of interest in the sample gas is determined based on the reduction in the light intensity caused by the absorption of the light at the point of a selected absorption line of the gas component, as explained in DE 102011079342 B3. In this case, the laser diode is actuated periodically corresponding to a predetermined current-time function, such as a current ramp, in order to sample the absorption line of the gas component in a wavelength-dependent manner. In addition to the current, the temperature of the laser diode to a strong degree also determines the intensity and wavelength of the light generated, for which reason the laser diode is mounted on a heat sink with temperature regulation. Owing to aging of the laser diode, the optical power is reduced and the wavelength of the light generated changes, with the result that further measures for wavelength stabilization are required. For this purpose, for example, the laser diode is actuated in each actuation period with two different successive current-time functions to sample an absorption line of a reference gas as well as the absorption line of the gas component to be measured. The temperature of the laser diode or of the heat sink is then regulated via the position of the absorption line of the reference gas such that the absorption line is always located at the same point, preferably the center of the current-time function in question. The wavelength spacing of the absorption line is known. As a result, the absorption line of the gas component to be measured is also always at the same point in the current-time function sampling it.
In the conventionally used types of laser diodes, i.e., a vertical cavity surface-emitting laser (VCSEL) and a distributed feedback (DFB) laser, the frequency-determining component is a Bragg reflector. This consists of a sequence of thin layers of alternating refractive index. Some of the incident light is reflected at each interface, where the reflected rays in a wavelength that corresponds to a quarter of the optical thickness of the layers are superimposed on one another constructively. In the region around this central wavelength, the reflection is very high and decreases severely for relatively large and relatively small wavelengths. The dimensions of the Bragg reflector can be varied by changing the temperature, where the wavelength increases as the temperature increases. The temperature of the Bragg reflector is determined by the power loss of the laser diode and the temperature of the heat sink. The power loss of the laser diode is in turn dependent on its current-voltage characteristic, which can be described by component parameters, such as threshold voltage and bulk resistance.